The Fill In Signalling Unit acts as flag in TDM (Time Division Multiplexing) based SS7 (Signalling System No.7) signalling networks when there is no other information to send. The FISU consists of MTP3 (Message Transfer Part – Level 3) routing information and is sent in place of flags in idle periods. It also serves to verify the integrity of the link as the receiving side will performing a FCS (Frame Check Sequence) on the FISU.
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